It is known from the experiment [1] that in an excess of H2O2 with respect to Fe2+ ions catalytic decomposition of hydrogen peroxide to produce water and O2 accompanies the oxidation of Fe2+ ions. Available in the literature quantum chemical calculations investigating the nature of the oxidizing agent in the Fenton reaction, usually start from the [FeII(H2O)5H2O2] 2+ intermediate in which one water molecule is replaced by H2O2 [2]. In this work we study the following mechanism of H2O2 catalytic decomposition and the oxygen production in the Fenton chemistry For this purpose the density functional theory (DFT) calculations of [FeIIH2O)6] 2+ , [FeIV(H2O)4(OH)2] 2+ and H2O2 systems as well as the found transition states (TS) of two stages of catalytic cycle (scheme above) were carried out using the PRIRODA 06 program package [3]. The energy profiles of two reactions are presented in Figure 1. It is seen that both reactions are exothermic. Calculated activation energy of the first stage which corresponds to the O-O bond homolytic cleavage is equal to 14.99 kcal/mol, which is in reasonable agreement with the experiment.